Pigment dispersion and offset printing ink composition using the same

Abstract

A pigment dispersion is disclosed which comprises a pigment dispersant, a nonaqueous solvent and optionally a binder resin, wherein the pigment dispersion comprises as the pigment dispersion at least 0.2 parts by weight of a modified novolak resin (A) relative to 100 parts weight of the pigment, the modified novolak resin (A) having an aromatic ring and a ring structure given by ring opening of an epoxy group by a carboxyl group of a hydroxycarboxylic acid or a derivative thereof. An ink composition for offset printing containing the pigment dispersion is also disclosed.

What is claimed is: 
     
       1. A pigment dispersion comprising a pigment, a pigment dispersant, a nonaqueous solvent and optionally a binder resin; the pigment dispersion comprising as the pigment dispersant at least 0.2 part by weight of a modified novolak resin (A) relative to 100 parts by weight of the pigment,   the modified novolak resin (A) containing at least one group represented by formula (1) ##STR33## wherein the oxygen atom at the left end is derived from an oxygen atom of a phenolic hydroxyl group of a novolak resin; W 1  and X 1  are independently each a divalent hydrocarbon group having 1 to 19 carbon atoms; i and j are an integer of 1 to 30 and an integer of 0 to 30, respectively, and satisfy the following conditions: i or j≧2 and i+j≧2; and R 1  is a hydrogen atom or a methyl group.   
     
     
       2. The pigment dispersion of claim 1, wherein the modified novolak resin (A) further contains, in a molecule thereof, at least one group represented by formula (5): ##STR34## wherein the oxygen atom at the left end is derived from an oxygen atom of a phenolic hydroxyl group of the novolak resin; Y is a monovalent organic group having 1 to 20 carbon atoms and, at its bonding end, an oxygen atom or a nitrogen atom (excluding a group represented by formula (6): ##STR35## wherein W 1  and i each have the same definition as recited in claim 1); and R 10  is a hydrogen atom or a methyl group. 
     
     
       3. The pigment dispersion of claim 1 or 2, wherein the modified novolak resin (A) is intermolecularly or intramolecularly bridged by a crosslinking group represented by formula (7): ##STR36## wherein the oxygen atom at the right end is derived from an oxygen atom of a phenolic hydroxyl group of the same or different molecule of the novolak resin; Z is a divalent to hexavalent organic group having 1 to 40 carbon atoms and, at its bonding end, an oxygen atom or a nitrogen atom; k is an integer of 2 to 6; and R 11  is a hydrogen atom or a methyl group. 
     
     
       4. The pigment dispersion of claim 1 or 2, wherein the ratio of phenolic hydroxyl groups remaining in the modified novolak resin (A) is not higher than 5.0%. 
     
     
       5. The pigment dispersion of claim 1 or 2, wherein the modified novolak resin (A) contains no phenolic hydroxyl group and further contains, in a molecule thereof, at least one group represented by formula (8): ##STR37## wherein the oxygen atom at the left end is derived from an oxygen atom of a phenolic hydroxyl group of the novolak resin; and R 12  is a hydrogen atom or a methyl group. 
     
     
       6. The pigment dispersion of any of claim 1 or 2, wherein the group represented by formula (1) is a group represented by formula (12); ##STR38## wherein the oxygen atom at the left end is derived from an oxygen atom of a phenolic hydroxyl group of the novolak resin; W 3  and X 3  are independently each a divalent hydrocarbon group having 11 to 19 carbon atoms; i and j are an integer of 1 to 30 and an integer of 0 to 30, respectively; and R 1  is a hydrogen atom or a methyl group. 
     
     
       7. An ink composition for offset printing comprising a pigment dispersion, a binder resin and a lipophilic solvent; the pigment dispersion comprising a pigment, a pigment dispersant, a nonaqueous solvent and optionally a binder resin;   the pigment dispersion comprising as the pigment dispersant at least 0.2 part by weight of a modified novolak resin (A) relative to 100 parts by weight of the pigment,   the modified novolak resin (A) containing at least one group represented by formula (12): ##STR39## wherein the oxygen atom at the left end is derived from an oxygen atom of a phenolic hydroxyl croup of a novolak resin; W 3  and X 3  are independently each a divalent hydrocarbon group having 11 to 19 carbon atoms; i and j are an integer of 1 to 30 and an integer of 0 to 30 respectively, and satisfy the following conditions: i or j ≧2 and i+j≧2; and R 1  is a hydrogen atom or a methyl group.   
     
     
       8. The ink composition of claim 7, wherein the modified novolak resin (A) further contains, in a molecule thereof, at least one group represented by formula (5): ##STR40## wherein the oxygen atom at the left end is derived from an oxygen atom of a phenxolic hydroxyl group of the novolak resin, Y is a monovalent organic group having 1 to 20 carbon atoms and, at its bonding end, an oxygen atom or a nitrogen atom (excluding a group represented by formula (15): ##STR41## wherein W 3  and i each have the same definition as recited in claim 10); and R 10  is a hydrogen atom or a methyl group. 
     
     
       9. The ink composition of claim 7 or 8, wherein the modified novolak resin (A) is intermolecularly or intramolecularly bridged by a crosslinking group represented by formula (7): ##STR42## wherein the oxygen atom at the right end is derived from an oxygen atom of a phenolic hydroxyl group of the same or different molecule of the novolak resin; Z is a divalent to hexavalent organic group having 1 to 40 carbon atoms and, at its bonding end, an oxygen atom or a nitrogen atom, k is an integer of 2 to 6; and R 11  is a hydrogen atom or a methyl group. 
     
     
       10. The ink composition of claim 7 or 8, wherein the ratio of phenolic hydroxyl groups remaining in the modified novolak resin (A) is not higher than 5.0%. 
     
     
       11. The ink composition of claim 7 or 8, wherein the modified novolak resin (A) contains no phenolic hydroxyl group and further contains, in a molecule thereof, at least one group represented by formula (8). ##STR43## wherein the oxygen atom at the left end is derived from an oxygen atom of a phenolic hydroxyl group of the novolak resin; and R 12  is a hydrogen atom or a methyl group. 
     
     
       12. The pigment dispersion of claim 1, wherein W 1  and X 1  are independently each a divalent hydrocarbon group having 1 to 10 carbon atoms. 
     
     
       13. The pigment dispersion of claim 1, wherein W 1  and X 1  are independently each a divalent hydrocarbon group having 5 to 10 carbon atoms. 0 to 30 respectively and satisfy the following conditions: i or i ≧2 and i+j≧2; and R 1  is a hydrogen atom or a methyl group. 
     
     
       14. The pigment dispersion of claim 1, wherein the modified novolak resin is derived from an aldehyde and a monohydric phenol or a polyhydric phenol. 
     
     
       15. The pigment dispersion of claim 7, wherein the modified novolak resin is derived from an aldehyde and a monohydric phenol or a polyhydric phenol.